This application claims the priority of Japanese application 10-184533, filed Jun. 30, 1998 in Japan, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to an air flow rate measurement device used for control of an automobile engine.
An air flow meter with a heat generating resistor is one of the devices which detect intake air flow rate of an internal combustion engine. The air flow meter with a heat generating resistor is designed in such a manner that the heat generating resistor disposed in an air flow passage is heated up to a predetermined temperature and the intake air flow rate is detected based on a heating current supplied to the heat generating resistor.
An example of such air flow meters with a heat generating resistor is an intake air flow rate detecting device disclosed in JP-A-2-85724(1990). The known intake air flow rate detecting device is provided with a correction circuit, in other words a ratiometric circuit which corrects an output signal from an intake air flow rate detecting circuit depending on variation of a reference voltage supplied to an A/D converter circuit and outputs the same to the A/D converter circuit, and with the provision of the ratiometric circuit, an intake air flow rate detecting device is realized which permits an accurate intake air flow rate detection regardless of a possible variation of the reference voltage supplied to the A/D converter circuit.
However, in the known intake air flow rate detecting device when current flowing through the heat generating resistor increases, the voltage drop at the reference potential (GND) increases due to the wiring resistance, a ground potential Vg serving as a reference potential for the ratiometric circuit becomes higher than a ground potential Vgnd serving as a reference potential for an engine control unit (ECU), and the amplification rate of the ratiometric circuit decreases, thereby a problem is posed which causes errors and reduces accuracy in the output signal of the intake air flow rate detecting device.
FIG. 5 shows a schematic circuit diagram of the above explained conventional ratiometric output type air flow meter 50 with a heat generating resistor.
In FIG. 5, the collector of a transistor 60 is connected to a power source and the emitter thereof is grounded via a heat generating resistor 2 and a current detecting resistor 1. Numeral 5 denotes a wiring resistance between the current detecting resistor 1 and the ground. Further, the junction point between the heat generating resistor 2 and the current detecting resistor 1 is connected to a non-inverted input terminal of an operational amplifier 61, and an inverted input terminal of the operational amplifier 61 is connected to an output terminal thereof.
Still further, the output terminal of the operational amplifier 61 is connected to one of two input terminals of a multiplier circuit 11 in a ratiometric unit 10. The other input terminal of the multiplier circuit 11 is connected to the junction point between invariable resistors 3 and 4 connected in series with each other. The invariable resistor 3 is grounded via the invariable resistor 4 and the wiring resistance 5, and the invariable resistor 3 is supplied with an external reference voltage Vref for an engine control unit (ECU) 20. An output Vout of the multiplier circuit 11 is supplied to the ECU 20.
Although not illustrated in FIG. 5, a set of resistors is connected in parallel with the heat generating resistor 2 and the current detecting resistor 1 so as to constitute a bridge circuit. Therefore, when the bridge circuit imbalances due to increase in air flow rate, an output signal from an operational amplifier (not shown) for the bridge circuit, which is designed to sense such increase, is supplied to the base of the transistor 60 to supply a current to the heat generating resistor 2. In this instance, the larger the flow rate increase is, the supply current has to be increased accordingly; therefore, through the detection of output voltage V0 at the operation amplifier 61 which corresponds to the supply current, the air flow rate can be measured.
Now, when the grounding potential Vgnd for the ECU 20 in the device shown in FIG. 5 is used as the reference, because of the existence of the wiring resistance 5 between the ratiometric output type air flow meter 50 with a heat generating resistor and the ECU 20, when current Ia flowing through the heat generating resistor 2 increases, the reference grounding potential Vg of the ratiometric circuit 10 becomes higher than the reference grounding potential Vgnd.
The output voltage Vout of the ratiometric circuit 10 is expressed as follows, wherein it is assumed that the voltage proportional to the current flowing through the heat generating resistor 2 is V0, the proportion constant thereof is A and the external reference voltage for the circuit is Vref;
Vout=Axc3x97V0xc3x97(Vrefxe2x88x92Vg)xe2x80x83xe2x80x83(1)
Because of the differences in the reference grounding potentials, an error Err is caused in the output voltage Vout as expressed by the following equation (2);
Err=xe2x88x92Axc3x97V0xc3x97Vgxe2x80x83xe2x80x83(2)
Since the reference ground potential Vg corresponds to a voltage drop Iaxc3x97R5 which is caused when the heating current Ia for the heating resistor 2 flows through the wiring resistance 5 (wherein the resistance value thereof is assumed as R5), and the heating current Ia is proportional to a voltage V2 detected by the current detection resistor 1, the reference ground potential Vg is proportional to V2xc3x97R5.
Further, since the air flow rate detection voltage V0 is also proportional to the heating current Ia and the heating current Ia is proportional to the voltage V2 and wherein it is assumed the proportion constant thereof is B, the equation (2) can be expressed by the following equation (3), wherein the error Err is expressed by a quadratic function of the voltage V2;
Err=xe2x88x92Bxc3x97(V2)2xc3x97R5xe2x80x83xe2x80x83(3)
Even if the error Err expressed by the quadratic function of the voltage V2 is attempted to be adjusted by a characteristic adjusting means making use of a DC amplifier circuit, the correction of the error is difficult and the problem as referred to above is posed that the accuracy in the output signal of the air flow meter is reduced.
An object of the present invention is to provide a ratiometric output type air flow meter with a heat generating resistor which permits a highly accurate air flow rate measurement regardless of the variation in the ground potential caused by the current flowing through the heat generating resistor.
In order to achieve the above object, the ratiometric output type air flow meter with a heat generating resistor in which the heat generating resistor is disposed in an air flow passage and is supplied with a heating current to heat the same up to a predetermined temperature and which includes a ratiometric circuit converting an air flow rate signal V0 detected based on the heating current to a signal proportionated to an external reference voltage Vref and outputting the same, comprises a correction circuit which is designed to correct the conversion error through the ratiometric circuit which is caused by the heating current flowing through the wiring for grounding.
When the current flowing through the heat generating resistor increases, the reference potential drop due to the wiring resistance increases accordingly, thereby, the ground potential Vg serving as the reference potential for the ratiometric circuit becomes higher than the ground potential Vgnd serving as the reference potential for an engine control unit (ECU). Therefore, the amplification rate of the ratiometric circuit decreases, causing errors therein and reduced accuracy in output signals of the concerned air flow meter reduces.
Accordingly, when the conversion errors in the ratiometric circuit are corrected with the correction circuit, the accuracy reduction in the output signals of the air flow meter is prevented.
Preferably, in the above described ratiometric output type air flow meter with a heat generating resistor, the ratiometric circuit comprises a divider circuit which divides the external reference voltage Vref to obtain an input voltage Vex and a multiplier which receives the input voltage Vex and the air flow rate signal V0 as the input signals thereof and outputs an output signal Vout as the output of the ratiometric circuit, and the correction circuit is connected in series to the divider circuit at the side of the reference potential in the ground wiring of the air flow meter and includes a correction use resistor for correcting a voltage drop due to the heating current flowing through the ground wiring.
Further preferably, in the above described ratiometric output type air flow meter with a heat generating resistor the ratiometric circuit comprises a divider circuit which divides the external reference voltage Vref to obtain an input voltage Vex and a multiplier which receives the input voltage Vex and the air flow rate signal V0 as the input signals thereof and outputs an output signal Vout as the output of the ratiometric circuit. The correction circuit is connected between the division point in the divider circuit generating the voltage Vex and an input terminal of the multiplier where the air flow rate signal V0 is inputted and includes a correction use resistor for correcting a voltage drop due to the heating current flowing through the ground wiring.
Further preferably, in the above explained ratiometric output type air flow meter with a heat generating resistor, the ratiometric circuit comprises a divider circuit which divides the external reference voltage Vref to obtain an input voltage Vex and a multiplier which receives the input voltage Vex and the air flow rate signal V0 as the input signals thereof. The correction circuit is connected in series to the divider circuit at the side of the reference potential in the ground wiring of the air flow meter and includes a correction use variable resistor which can adjust a linearity of the output signals of the air flow meter by varying the resistance value thereof and an output characteristic adjusting circuit disposed at the output terminal of the ratiometric circuit and for adjusting the output characteristics of the air flow meter.
Further preferably, in the above described ratiometric output type air flow meter with a heat generating resistor, the ratiometric circuit comprises a divider circuit which divides the external reference voltage Vref to obtain an input voltage Vex and a multiplier which receives the input voltage Vex and the air flow rate signal V0 as the input signals thereof. The correction circuit is connected between the division point in the divider circuit generating the voltage Vex and an input terminal of the multiplier where the air flow rate signal V0 is inputted and includes a correction use variable resistor which can adjust a linearity of the output signals of the air flow meter by varying the resistance value thereof and an output characteristic adjusting circuit disposed at the output terminal of the ratiometric circuit and for adjusting the output characteristics of the air flow meter.